Youth - and size - won out over experience in a ring called the Octagon.

Brock Lesnar, a 31-year-old former NCAA wrestling champ, pummeled Randy Couture, 45, to the mat at 3:07 of the second round to win Ultimate Fighting's ultimate championship, the heavyweight title, last night at the MGM Grand Garden Arena in Las Vegas.

Aside from the disparity in size - Lesnar is a hulk at 6 feet 3 inches, 275 pounds compared with Couture, 6-1 and a lean 220 - Couture hadn't fought in 15 months, disdaining contract squabbles as he awaited the big fight he craved.

And big is what he got.

Lesnar, who lost the 1999 NCAA wrestling title to the Patriots' Stephen Neal, then won the title the next season, had the upper hand throughout the fight. He landed a standing elbow that stunned Couture to open Round 2 and had the defending champion backpedaling.

Lesnar landed a right hand to Couture's temple, then engulfed him with hammer punches with Couture (16-9) on the mat. The referee had to step in and stop the fight.

Lesnar, billed as one of the sport's rising young stars, earned a technical knockout to win UFC 91 in just his fourth pro fight.

The UFC was hoping to draw the biggest pay-per-view audience in the sport's young history. Chuck Liddell's fight with Tito Ortiz drew 1.05 million buys in December 2006.

"It sounds really good," Lesnar said after being introduced as the new heavyweight champion. "I can't believe it.

"I just believe in hard work and it pays off."

Lesnar is an interesting story in a sport trying to cut into the mainstream audience.

He gained prominence as the youngest champion in the history of World Wrestling Entertainment, then left the WWE in 2004 to pursue a career in the NFL - perhaps spurred on by Neal's success with the Patriots - but he was cut by the Minnesota Vikings.

Lesnar began his fighting career in June 2007, with Mixed Martial Arts, then signed with UFC in October. It took him a little more than a year to win the sport's biggest title.